In addition to new lights, when I see white spots like that a few things come to mind:
-Super low nutrients (ie. excessive biopellets). Basically it's starving. But seeing that the other corals around it are not super hot, I don't think this is the issue.
-Alk fluctuations. How are you topping up your calc and alk? It looks like the swings might be too large.
-New GFO. I've had a couple of colonies go belly up on me if I change too much GFO at once. The process by which they depart sound similar to what you are seeing and takes a few days to a week. The GFO, in addition to dropping phosphates to very low levels, can cause other parameter fluctuations which can take out more sensitive corals.
Based on what I see there, I'm putting my money on a parameter fluctuation; probably alk but it could be any one of them.
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